1. Field of the Invention
The present disclosure relates to microprocessors and, in particular, to microprocessors used in secure systems such as Pay-TV decoders known generally as set-top-boxes.
2. Description of the Related Art
A conditional access device, which restricts access to data, such as a Pay-TV decoder, typically comprises a processor, cryptographic circuits, memory (such as random access memory or RAM) and inputs and outputs for receiving, decoding and outputting decoded television signals.
We have appreciated the problem of checking that programs that are executed by the processor are authorized and are not “hacked” code. We have further appreciated that simply checking the authenticity of program code in RAM may not be effective because, when executing, code is transferred to cache memory. Checking code every time it is executed from RAM may, therefore, fail to prevent execution of unauthorized code and, in any event, would degrade the performance of the processor.